In a solid, the atoms are tightly bound together. Solids have a fixed volume and shape. Heating up a solid will often cause it to change to a liquid. Liquids have atoms that are less tightly bonded together. They have a fixed volume, but not a fixed shape. Adding heat energy to a liquid will often transform it to a gas. Gasses have neither a fixed volume nor a fixed shape. Their atoms are not bonded together and they move around freely.

When heat energy is added to a gas, it begins to move around faster. When enough heat energy is added to a gas it can become a plasma.

Plasma is similar to gas, but its particles have a different structure and charge. Plasma forms when a gas becomes extremely hot. When this happens, the gas’ atoms gain lots of energy. This energy causes the electrons to detach from the nuclei of the gas’ atoms. When the negatively charged electrons detach, the positively charged protons and neutral particles called neutrons in the nuclei are left. These positively charged nuclei are called ions. When a gas is so hot that the electrons and protons split apart to form electrons and ions, we say that the gas has been ionized. Plasma is an ionized gas.